How to create a minimap with independent movement?
Moving map camera

The code snippet does not move the camera at all. 

I guess it is supposed to setup the VideoTexture source of an object that shows the output of the map camera.

You simply move the map camera as you like. I guess you break the parent-child relationship to the character to achieve what you are looking for.

Finding the minimap camera

The code snippet contains a strange way to find the minimap camera.

I suggest following search:

for scene in logic.getSceneList():
    mapcam = scene.cameras.get("map_cam")


Edit:

You already split the task down into smaller operations. This is good. 

My above answer was telling you how to improve your code snippet, which btw. does not do any of the described operations. So I'm a bit lost where your problems are.

I suggest to discuss that in a forum like blenderartist.org or ask single more specific questions.

Here is a short strip down of the mentioned operations. 

move camera only

As easy as it sounds. Do not parent it to something. Apply logic to move the camera.

camera travel a long the mini map shows the map in a litle circle in a corner of the screen

Do you mean you want to minimap shown in an edge of the window?




Place a mesh object (disc) there. 
Add UV and a material with a texture
apply the videotexture as you mentioned in your snippet to show the render of the minimap camera. (You might want to use the DynamicTexture project if you are not that familar with Python)
refresh the texture at each single frame


without Character

This depends on what the minimap camera is seeing. 

Typically you do not show the same thing as the main camera. You show a corresponding "level" that contains a map (several objects that form the map).



The minimap camera gets moved over this map. As result the minimap object (see section above) will show different parts of the map (rather than the level). As the map does not contain the character, it will not be visible.

The trick is to know how to move the minimap camera to get a correlation to the level camera.

press a button to back to start place

Play an action that set the minimap camera to that start place (above the map)

parent camera again to the character and walk with the camera follow the character

Parenting is an option. This will work fine when 


the level is the map
the map is at an offset of the level and has the same scale as the level (e.g it is under the ground. 


Example: the map is under the terrain:


(side view)

It will not work fine when
* the map is in a different scene
* the map has a different scale or orientation

In such cases you need Python to synchronize the motion of the minimap camera with the motion of the character.